We have constructed various baculovirus recombinants expressing selected rotavirus proteins including VP4 (P1A, P1B, P2, P3, P4, P5B, P6 or P7), VP7 (G1, G2, G3, G4, G6 or G9) or NSP4 (genotype A, B, C or D). Previously, by using immunocytochemistry assay involving such recombinants, we analyzed sera obtained from gnotobiotic calves or piglets infected orally with homologous bovine (NSP4[A]) or porcine (NSP4[B]) virus. We reported that following primary infection and subsequent challenge with virulent rotaviruses, naive calves and piglets developed either higher or significantly higher IgA and/or IgG antibody titers to the homologous-host homotypic NSP4[A] or [B] proteins than those to the heterologous-host homotypic or heterologous host heterotypic rotavirus NSP4s, indicating that primary and secondary antibody responses were species-specific rather than genotype-specific. Unexpectedly, such isotype antibody responses were shown to be correlated closely with the molecular phylogenetic relationships of NSP4 proteins within a species-specific region of amino acids 131-141, reaffirmng that NSP4-specific antibody responses were primarily species-specific. In piglets, antibodies to NSP4 induced by previous oral infection failed to confer protection against oral challenge from a porcine rotavirus bearing serotypically different VP4 and VP7 but essentially identical NSP4 to the porcine rotavirus in primary infection. Thus, as a potential approach to immunization with a live oral rotavirus vaccine, the NSP4 protein did not appear to be important in protection against rotavirus disease and infection. Previously, a total of 18 adult volunteers was given orally 1 ml of a 0.2% stool filtrate containing the virulent human rotavirus strain D (G1P1A[8], NSP4[B]) (Kapikian, et al., 1983. J. Infec. Dis. 147:95). Four of 5 who shed rotavirus after challenge developed diarrhea. In an attempt to identify correlates of resistance to rotaviral diarrheal disease and/or infection, we analyzed serum IgA and IgG antibody titers in 16 of the 18 volunteers. We used an immunocytochemistry assay involving a total of 18 different recombinant baculoviruses expressing each of the following major serotype/genotype of rotavirus proteins for the serologic assays: VP4 (P1A[8]; P1B[4]; P2A[6]; P3[9]; P4[10]); VP7 (G1-4, G9); NSP4 ([A]-[D]); as well as VP6 and NSP2. The prechallenge IgG antibody titers to types G1 and G3 VP7, types P1A[8] and P2A[6] VP4 and type [A] NSP4 in the non-infected group were significantly higher than those in the asymptomatically infected and symptomatically infected groups. Logistic regression analysis showed that the higher probability of resistance to asymptomatic and symptomatic rotavirus infection correlated with higher prechallenge IgG antibody titers to the homotypic VP7 (G1) (p=0.0077 for asymptomatic infection; p=0.0004 for symptomatic infection) and the homotypic VP4 (P1A[8]) (p=0.0091 for asymptomatic infection; p=0.0011 for symptomatic infection), suggesting that protection against rotavirus infection and disease may be VP7- and/or VP4-homotypic although they may not be the sole determinants of protective immunity.